Detection of reactive oxygen species generated by microwave electrodeless discharge lamp and application in photodegradation of H 2 S

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1 Korean J. Chem. Eng., 30(7), (2013) DOI: /s z INVITED REVIEW PAPER Detection of reactive oxygen species generated by microwave electrodeless discharge lamp and application in photodegradation of H 2 S Yang Yu, Tingting Zhang, Liqiong Zheng, and Jiang Yu College of Chemical Engineering, Beijing University of Chemical Technology, Beijing , P. R. China (Received 30 December 2012 accepted 4 May 2013) Abstract The photodegradation of hydrogen sulfide (H 2 S) was examined using a self-made microwave electrodeless discharge lamp (MEDL). The features of the MEDL had been tested. The results showed that the MEDL absorbed 18.3, 32.7 and 41.8 W power at the microwave (MW) output power of 165, 330 and 660 W, respectively. The intensity of the emitted light increased with increasing MW output power. The reactive oxygen species (ROS) generated by irradiated air and nitrogen were detected, respectively. It was illustrated that the irradiated air could generate a number of ROS, at least including 1 O 2 and OH. And the amount of ROS increased with increasing MW output power. In photodegradation of H 2 S process, the effects of MW output power and gas composition were investigated. The removal efficiency of H 2 S under nitrogen was obviously lower compared with that under air. The removal efficiency of H 2 S increased with increasing MW output power. Key words: Microwave Electrodeless Discharge Lamp (MEDL), Reactive Oxygen Species (ROS), Photodegradation, Hydrogen Sulfide INTRODUCTION Hydrogen sulfide (H 2 S) is a representative odorous pollutant released as a by-product of many processes, such as sour gas flaring, petroleum refining, pulp and paper manufacturing or wastewater treatment [1-3]. H 2 S has an extremely low odor threshold (0.5 ppb) [4]; not only it can cause corrosion of expensive metal parts, but also can threaten human safety [5,6]. Therefore, a cost-effective technology for treatment of reduced H 2 S is highly desired. Many studies have been conducted on the photocatalytic oxidation of H 2 S at low concentration using TiO 2 [7-9]. Compared to other techniques, the photocatalytic oxidation of H 2 S offers several advantages, such as low cost, energy saving and minimal waste. However, very few commercial applications of this technology are available at present due to deactivation of the catalyst and low quantum efficiency. During the course of the photocatalytic reaction, the byproducts and products accumulated on the surface of TiO 2 could cause the deactivation of the catalyst [7,10]. Most of the photoinduced positive holes (h + ) and electrons (e ) had recombined before they were trapped by hydroxyl or oxygen and quantum efficiency was usually less than 5% [11]. To enhance the quantum efficiency of the photocatalytic oxidation, microwave (MW) energy was used to assist photocatalytic reactions for MW radiation could somehow affect the activity of the TiO 2 particle surface. In this context, microwave electrodeless discharge lamp (MEDL) has been used in many photocatalytic processes as illuminant [12-15]. When placed into the MW field, the MEDL will generate UV-Vis radiation by absorbing the MW power. And at the same time, the MW field interacts with the reaction mixture [16,17]. Therefore, the application of MEDL brings the simultaneous effect of both UV-Vis and MW irradiations on photochemical reactions. At present, this technique is mainly used to photo oxidize organic pollutants in aqueous solution. Nevertheless, the photodegradation of inorganic compounds in gaseous phase using MEDL as illuminant has been little explored. In an interesting work developed by Xia et al. [18], an MEDL was used for the photodegradation of H 2 S. They observed high removal efficiency. It is worth mentioning that they did not use any photocatalyst, so there was no problem of the deactivation of catalyst. No doubt, the photodegradation of H 2 S using MEDL is promising. In fact, the reactive oxygen species (ROS) may play a very important role during the photodegradation of H 2 S with MEDL. However, the effects on the generation of ROS with MEDL in gaseous phase are not well understood. Besides, MW output power and atmosphere should affect the photodegradation process, but these effects have not been well investigated. In this study, we examined the characteristic features of the selfmade MEDL in terms of the absorbed power and spectra. The method of oxidation-extraction spectrometry (OES) with application of 1,5- diphenyl carbazide (DPCI), which has been used to determine the ROS in aqueous solution [19,20], was used to detect the ROS generated by MEDL in gaseous phase. That is, DPCI can be oxidized by ROS into 1,5-diphenyl carbazone (DPCO), which can be extracted by the mixed solution of benzene and carbon tetrachloride and show great absorbance at 563 nm wavelength. Three kinds of radical scavengers were used to determine the kinds of ROS generated by MEDL. The effects of operating parameters, including MW output power and the atmosphere on the photodegradation of H 2 S were evaluated. And last, we examined the final products to explore the possible mechanism of photodegradation of H 2 S. EXPERIMENTAL SECTION To whom correspondence should be addressed. yujiang@mail.buct.edu.cn, yuyangbuct@126.com Materials Benzene and carbon tetrachloride were obtained from Beijing

2 1424 Y. Yu et al. Table 1. MW energy absorbed by the water and beaker MW output energy (W) Pa a (W) Pb b (W) Pb-Pa (W) a With the MEDL in the MW field b Without the MEDL in the MW field Fig. 1. (a) Configuration of the photochemical reactor: (1) quartz pipe (2) MEDL; (b) a photo of the MWP reactor with a working MEDL. Chemical Works. L-histidine (His) and Vitamin C (VC) were purchased from Beijing Aoboxing Bio-tech Co., Ltd. Thiourea (TU) was purchased from Tianjin GuangFu technology development Co., Ltd. 1,5-diphenyl carbazide (DPCI) was obtained from Tianjin Fuchen Chemistry Works. All chemicals used were analytical grade. 2. The Microwave Photochemical (MWP) Reactor Fig. 1(a) gives the configuration of the photochemical reactor, which consists of an MEDL and a quartz pipe. The MEDL is made of a quartz tube, filled with Hg (13.6 mg) and Ar (0.5 kpa), and sealed up at both ends. The dimension of the MEDL is 10 mm (external diameter) by 140 mm (length), but the middle of the MEDL shrank. A quartz pipe with the length of 250 mm and inner diameter of 25 mm is served as photochemical area with the MEDL in it. There is a small quartz pipe at each end of the pipe, and it is connected to a silicone tube through which the gas flows into the photochemical reactor. The photochemical reactor is placed in the MW oven (Qingdao MKW Microwave Applied Technology Co., Ltd.; power, 0-1,000 W; frequency, 2.45 GHz) comprising a microwave photochemical (MWP) reactor. The MEDL emitted UV-Vis light under a continual MW radiation. Fig. 1(b) is a photo of the MWP reactor with a working MEDL. 3. Characterization of the MEDL In this study, the microwave output power was adjusted to 165, 330 and 660 W, respectively. The power of the MEDL was measured by the method as Zhang presented [21,22]. A beaker of water was placed in the MW oven. MW radiation could raise the temperature of the water and the beaker. The MW power absorbed by the water and the beaker can be calculated by the following equation [23]: T P = ( c w m w + c b m b ) t where P is the power of MW absorbed by the water and beaker, m w and m b are the mass of water and beaker (water: g, beaker: g), c w and c b are the heat capacities of water and beaker (water: 4.18 J/g o C, beaker: 0.67 J/g o C), T is the temperature difference ( o C) between before MW radiation and after MW radiation, and t is the MW radiation time (s). The temperature of the beaker is equal to the temperature of the water in the beaker. With the MEDL in July, 2013 (1) Fig. 2. Schematic diagram of the generation and trap of ROS. 1. N 2 gas cylinder 5. Photochemical reactor 2. Air blower 6. MW oven 3. Reducing valve 7. DPCI solution 4. Mass flow controller the MW field, the MW energy absorbed by the water and beaker is defined as Pa. Without the MEDL in the MW field, the MW energy absorbed by the water and beaker is defined as Pb. Pa and Pb at different MW output powers are listed in Table 1. Because some microwave energy was absorbed by the MEDL, Pa was lower than Pb. So we assume that the power of the MEDL is Pb-Pa. The UV-Vis spectra of the MEDL were recorded by an MSB3000 spectrometer (Beijing JiKe Instrument Co., Ltd.). 4. Determination of Reactive Oxygen Species (ROS) The experimental setup designed for the generation and trap of ROS is shown in Fig. 2. The air stream (provided by an air blower, relative humidity of air was ca. 30% at room temperature) was irradiated by MEDL at MW output powers of 165, 330 and 660 W, respectively. The N 2 stream (99.9%; Beijing Zhongkehuijie Analysis Technology Company) was irradiated by MEDL at MW output power of 660 W. Every irradiation experiment lasted for 60 min. All the above gas products were sampled into 100 ml DPCI solution ( mol/l). The gas flow rate was 2 L min 1. Meanwhile, the air without irradiation was also sampled into 100 ml DPCI solution at the same gas flow rate for 60 min. Then, from each sample, ml solution was taken exactly and extracted with mixed solution of benzene and carbon tetrachloride (volume ratio=1 : 1). All extracts were then diluted to 10.0 ml with the same mixture of benzene and carbon tetrachloride solvent and their UV-Vis spectra were recorded by a spectrophotometer (Beijing Persee Co., Ltd., TU-1901). 5. Determination of the Kinds of ROS To detect the kinds of ROS generated by MEDL, three kinds of scavengers were used to quench the different ROS. As is well known, TU can quench the hydroxyl radicals ( OH), and His can quench the singlet oxygen ( 1 O 2 ), while the VC can quench most kinds of ROS [24-26]. Firstly, four 25 ml DPCI stock solutions (

3 Reactive oxygen species for removal of H 2 S 1425 The amount of sulfate ion product was analyzed by an ion chromatograph (IC) (Dionex, ICS-900). Sixty liters of H 2 S sample gas with air/nitrogen (ca. 8.5 mg/m 3 ) were photodegraded, respectively. All the above gas products were introduced into 40 ml distilled water. The IC analysis was then carried out for determining the sulfate ion contents. In addition, a gas chromatograph (GC) (Shanghai Hengping Co., Ltd., GC-1120) with thermal conductivity detector (TCD) was used to analyze H 2 production during the photodegradation of H 2 S. RESULTS AND DISCUSSION Fig. 3. Schematic diagram of photodegradation of H 2 S. 1. N 2 gas cylinder 07. Surge flask 2. Air blower 08. Glass syringe 3. H 2 S/ N 2 gas cylinder 09. MW oven 4. Reducing valve 10. Photochemical reactor 5. Mass flow controller 11. Sulfur analyzer 6. Mixed gas flask 12. Alkaline trap 1. Characteristics of the MEDL As shown in Table 1, the MEDL irradiated with 165, 330 and 660 W of MW output power presented 18.3, 32.7 and 41.8 W of absorbed power, respectively. It indicates that the MEDL power increases with increasing MW output power, but there is not a linear relationship between them. The conversion process from MW energy mol/l) were added into four 100 ml volumetric flasks. Ten milliliters of stock solutions (0.05 mol/l) of His, VC and TU were added into above three volumetric flasks, respectively. All four solutions were diluted to 100 ml with distilled water and transferred into four conical flasks, respectively. Then, the air stream was irradiated by MEDL at the MW output power of 165 W for four times (60 min for each time). The gas products were sampled into the above four solutions, respectively. From each sample, ml solution was taken exactly and extracted with mixed solution of benzene and carbon tetrachloride (volume ratio=1 : 1). Finally, all extracted solutions were diluted to 10.0 ml with the same mixed benzene-carbon tetrachloride solvent and their UV-CVis spectra were determined. 6. Photodegradation of H 2 S The experimental setup for photodegradation of H 2 S, shown in Fig. 3, consists of three parts with a gas generation system, an MWP reactor, and a gas analysis system. Two kinds of gas samples were generated by mixing standard gas of H 2 S (1%, N 2 as balance air, V/V; Beijing Beiwen Company) with air and N 2, respectively. The reactant gas mixture was purged into the photochemical reactor in the absence of UV-Vis illumination until a steady state was established. Then, the MW oven was powered, and the MEDL was illuminated. After a new steady state was established (after 3 min of exposure to light), a trace sulfur analyzer (Chengdu Tianyu Co., Ltd., WDL-94) was used to analyze the H 2 S concentration before and after passing through the photochemical reactor. The gas residence time was 3.35 s, and the initial concentration of H 2 S changed from 4 to 16 mg/m 3. They were controlled by mass flow controllers. The H 2 S removal rate is calculated by the following equation: H 2 S removal rate (%) = effluent [ H 1 2 S] inf luent [ H 2 S] 7. Analysis of Photodegradation Process X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific, ESCALAB 250) was used to analyze S2p region of the white sediments on the inner pipe that were generated during the photodegradation of H 2 S in air atmosphere. (2) Fig. 4. UV-Vis spectroscopy of the MEDL radiated by MW at output power of 660 W. Fig. 5. Comparisons of the Hg emission line intensities with different MW output power. Korean J. Chem. Eng.(Vol. 30, No. 7)

4 1426 Y. Yu et al. Fig. 7. Absorbance of DPCO at 563 nm as function of various quenching reagents ([DPCI]= mol/l, [His]=[VC]=[TU] = mol/l). to MEDL UV-Vis is complex, and the conversion efficiency is very low. The UV-Vis spectra emitted from the MEDL under MW output power of 660 W are shown in Fig. 4. The atomic Hg emission lines for the MEDL appeared at 254, 313, 366, 406, 436, 509 and 546 nm. The MEDL showed the maximal emission at 254 nm assigned to 6 2 P S 0 of Hg atom [18]. The energy of HS-H is kj mol 1, corresponding to the emission line at 309 nm. Therefore, the Hg emission line at 254 nm emitted from the MEDL has enough energy to break HS-H. Fig. 5 gives the relationship of the Hg emission line intensity and MW output power. It was found that high MW output power could generate higher intensity Hg emission lines. 2. Generation of ROS by MEDL After absorbing energetic hv emitted from MEDL, O 2 /H 2 O etc. molecules in air atmosphere are able to generate ROS such as OH, O 2 and 1 O 2 and so on[18]. It has been reported that the oxidationextraction spectrometry (OES) method could be used to detect the ROS [19,20]. Because of the strong oxidation ability, the ROS can oxidize 1,5-diphenyl carbazide (DPCI) into 1,5-diphenyl carbazone (DPCO). The latter can be extracted by the mixed solution of benzene and carbon tetrachloride and show an obvious absorbance at 563 nm wavelength. Sequentially, the product and output of ROS can be detected easily. Fig. 6 displays that the absorption peaks of DPCO in DPCI solutions after introducing irradiated air at 563 nm all show an obvious increase, compared with the corresponding one after introducing pure air. This phenomenon indicates, under UV-Vis irradiation of the MEDL, the air can generate ROS, which causes the oxidation of DPCI forming DPCO. And that the DPCO in DPCI solution after introducing irradiated N 2 does not exhibit more obvious absorbance than the corresponding one that after introducing pure air. It is clear that there is no ROS generated in N 2 atmosphere. Moreover, the absorbance of DPCO in DPCI solutions after introducing irradiated air at 563 nm increases with the increase of MW output power. It indicates that the quantities of ROS increase with the increase of MW output power. 3. The Kinds of ROS Generated During Photochemical Reaction To explore the possible mechanisms of the photochemical process, it is important to detect the kinds of ROS. Here, three radical scavengers were used for this purpose. Through the quenching resulting from different scavengers, the kind of ROS can be judged. Fig. 7 gives the effect of introduction of scavengers on DPCO absorbance. Without any scavenger, the absorption peaks of DPCO in DPCI solutions after introducing irradiated air at 563 nm are very high. This indicates that a great amount of ROS is generated and a certain amount of DPCI is oxidized to DPCO. However, after adding quenchers, the absorbances of DPCO are weakened more or less. It implies that a number of ROS are consumed by the used scavengers, at least including 1 O 2 and OH. 4. Photodegradation of H 2 S Factors affecting the photodegradation process including MW output power, initial concentration of H 2 S and the atmosphere were investigated. Fig. 8 displays the photodegradation of H 2 S by MEDL under different conditions. Photodegradation of H 2 S under air atmosphere and nitrogen atmosphere was studied. The results are presented in line a and line d in Fig. 8, respectively. In nitrogen atmosphere, the removal efficiency Fig. 6. UV-Vis spectra of DPCO with different reaction conditions ([DPCI]= mol/l). Fig. 8. Removal of H 2 S in the photodegradation process under different conditions. Gas residence time in all experiments was 3.35 s. July, 2013

5 Reactive oxygen species for removal of H 2 S 1427 of H 2 S was lower than 7% with the initial concentration of H 2 S varying from 4 mg/m 3 to 9 mg/m 3, even reaching just 2% at the initial concentration of H 2 S of 10.8 mg/m 3. On the contrary, in air atmosphere with the same range of initial concentration of H 2 S, the removal efficiency of H 2 S was higher than 30%, even reaching up to 84% at the initial concentration of H 2 S of 4.37 mg/m 3. Therefore, the photodegradation of H 2 S was more effective in air atmosphere. The result of 3.2 well explains this phenomenon. It showed that the ROS was easily generated by irradiated air, while the ROS can oxidize H 2 S. Thus, we believe that in nitrogen atmosphere the removal of H 2 S was due to the HS-H being broken by the Hg emission line at 254 nm which could be absorbed by H 2 S molecules. The removal efficiency of H 2 S enhanced when in air atmosphere was mainly caused by the ROS. We consider the oxidation of H 2 S by the ROS is the major mechanism in the photodegradation process under air atmosphere. The effects of MW output power on photodegradation of H 2 S are shown in lines a, b, and c in Fig. 8, respectively. The removal efficiency of H 2 S increased dramatically with increasing MW output power. This was ascribed to the increase of MEDL power leading a higher light intensity to form more ROS. In view of application, it is important to investigate the effect of the initial concentration of H 2 S on the removal efficiency. The photodegradation of H 2 S under the MW output power of 660 W was performed with different initial concentrations of H 2 S in the range of 4 to 16 mg/m 3. The experimental results are shown in line a in Fig. 8. The removal efficiency significantly decreased with increasing the concentration of H 2 S from 84% at 4.37 mg/m 3 to 15% at 14.9 mg/m 3. The reason for higher initial concentration of H 2 S with lower removal efficiency can be described as follows. The number of H 2 S molecules increased along with increasing initial concentration. However, the light intensity and the amount of ROS in the photochemical reactor did not increase at the fixed MW output power. Consequently, the removal efficiency became low. 5. Reaction Mechanism of Photodegradation of H 2 S The products were investigated to explore the photodegradation mechanism of H 2 S. No yellow sediment was observed, which indicates that there is not any elemental sulfur produced under air or Table 2. Production concentration of SO 4 analysis via IC Sample gas MW output power (W) Inlet [H 2 S] (mg/m 3 ) SO 4 (mg/l) Pure air H 2 S+N H 2 S+air nitrogen. This implies that H 2 S cannot be decomposed to element sulfur by MEDL irradiation directly. But some white sediment was observed during the photodegradation of H 2 S in air atmosphere. Fig. 9 shows the S2p region of XPS spectrum of white sediments on the inner pipe. The peak at ev was due to the sulfate ion (SO 4 ). The peaks of elemental sulfur (S, ev) and sulfur dioxide (SO 2, ev) [4] were not observed. As shown in Table 2, the concentration of SO 4 in water that absorbed irradiated air (0.427 mg/l) is very small, and the SO 4 should come from water. The concentration of SO 4 in water that absorbed the products of photodegradation of H 2 S with air ( mg/l), in contrast, is very high. Thus, in air atmosphere, the H 2 S that is being irradiated can generate SO 4. The IC analysis showed that 4.63 µmol of SO 4 was formed during the photodegradation of 60 L of H 2 S sample gas (in an air atmosphere). Considering that the total volume of treated H 2 S and the measured H 2 S removal efficiency during the sampling period were 60 L and 41%, respectively, 6.18 µmol of H 2 S was eliminated at inlet H 2 S concentration of 8.5 mg/m 3. Consequently, 75% of the sulfur atoms of destructed H 2 S were recovered as sulfate in an air atmosphere. So the XPS and IC results suggest that the main product of photodegradation of H 2 S in air atmosphere is SO 4. On the other hand, the concentration of SO 4 in water that absorbed the products of photodegradation of H 2 S with N 2 does not show an obvious increase, compared with the corresponding one that absorbed irradiated air. It indicates that the H 2 S that is being irradiated in nitrogen atmosphere cannot generate SO 4. It s interesting that the water that absorbed the products of photodegradation of H 2 S with N 2 turned from colorless to yellow. It has been reported that photodegradation of sodium sulfide solutions that contain HS - without oxygen would generate S 2 and tetrasulfide [27]. Possibly, a good amount of HS ions were produced by MEDL irradiation with only N 2 as carry gas, and transferred to the soluble polysulfides in aqueous solution. Analysis of H 2 formation by the gas chromatograph shows, in air atmosphere, the formation of hydrogen was not detected during the photodegradation of H 2 S process. However, the formation of hydrogen was detected during the photochemical process in nitrogen atmosphere. Therefore, the mechanism of H 2 S photodegradation can be concluded as follows: (1) Photodegradation by MEDL irradiation in air atmosphere. H 2 S+hv HS +H + HS + 1 O 2 / OH intermediates SO 4 H 2 S+ 1 O 2 / OH intermediates SO 4 Fig. 9. X-ray photoelectron spectrum (XPS) of white sediments at S2p region during the photodegradation of H 2 S in air atmosphere. (2) Photodegradation by MEDL irradiation in nitrogen atmosphere. H 2 S+hv HS +H + Korean J. Chem. Eng.(Vol. 30, No. 7)

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